The RAD52 gene in Saccharomyces cerevisiae controls the repair of ionizing radiation-induced DNA double-strand breaks, radiation-induced spontaneous mitotic recombination, and recombination during meiosis. Utilizing an antibody raised against a Neurospora crassa deoxyribonuclease, we observed that in logarithmically growing wild type yeast strains, approximately 80% of Mg++ dependent pH 8 single-strand deoxyribonuclease is antibody precipitable. As cells enter stationary phase the antibody-precipitable nuclease decreases to an undetectable level as does cross-reacting material. The antibody precipitable nuclease activity increases five to ten times during meiosis during the period of DNA synthesis and recombination, and decreases at the end of meiosis. No activity is observed in rad50 or rad52 mutants; these results are correlated with the lack of recombination in these mutants. The nuclease that is detected with the antibody has been purified nearly 1000-fold and has a molecular weight of 70,000. It is a single-strand endo-exonuclease which also exhibits double-strand nuclease activity. Our observations implicate this exo-endonuclease in repair processes, spontaneous and damage-induced mitotic recombination, and normal meiotic recombination. Using a Lambdagt11 vector expression library that contains genomic yeast DNA and the antibody as the probe, we have identified a segment of DNA that codes for cross-reacting material. This will enable us to identify the gene and determine directly the role(s) of the nuclease in recombination and repair.